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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum-entangled photon interferometry

Roger K. Richards

Published Aug 2, 2004• 1 citation
Publisher

Abstract

A two-color quantum-entangled photon source is used to produce fourth-order interference. Because the period of the interference is produced by the frequency difference of the entangled photons, problems associated with counting fringes can be avoided. This also permits measurements at a virtual wavelength, which can prevent problems associated with transmission or absorption when such a longer wavelength may be needed. The interference wavelength can be varied with a geometry change in the beam path without any change in the source wavelength. The entangled photons are produced using an argon ion laser at 351 nanometers and a type I BBO crystal. The interference is detected in coincidence using four photomultiplier tubes.

DOI
10.1117/12.559687

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A two-color quantum-entangled photon source is used to produce fourth-order interference. Because the period of the interference is produced by the frequency difference of the entangled photons, problems associated with counting fringes can be avoided. This also permits measurements at a virtual wavelength, which can prevent problems associated with transmission or absorption when such a longer wavelength may be needed. The interference wavelength can be varied with a geometry change in the beam path without any change in the source wavelength. The entangled photons are produced using an argon ion laser at 351 nanometers and a type I BBO crystal. The interference is detected in coincidence using four photomultiplier tubes.
A new fourth-order interference experiment based on detecting photon pairs in coincidence has been carried out. A Mach-Zehnder type of interferometer is used, with the two photons generated simultaneously in the process of down-conversion as inputs and two photon counting detectors at the two outputs. The optical path difference can be varied piezoelectrically. It is found that the coincidence counting rate, after subtraction of accidentals, exhibits a sinusoidal variation with the optical path difference as predicted by quantum mechanics, but in violation of the laws of classical optics. This is another non-classical interference effeet. By contrast, two light beams derived from a He:Ne laser used as inputs to the interferometer, do not produce any fourth order interference.
Entangled photons and single photon interference are prime examples of quantum mechanics and are useful in evaluating the theory. To find entangled photons, an apparatus was built and optimized that used heralded detection. The apparatus produced entangled photons using a pump laser producing 474 nm radiation, and a ß-Barium Borate(BBO) crystal. The entangled 948 nm photons were produced in two separate beams arbitrarily known as signal and idler photons. Each beam was detected by an avalanche photodiode (APD). When the signal photons reached the APD, electronic pulses were produced and sent through a delay line, while electronic pulses from the idler photon APD were not. Then the pulses from the APDs were used as a start and stop for a time-to-amplitude converter (TAC). A multichannel analyzer was used to evaluate the TAC spectrum, and showed a large amount of coincidences at time intervals consistent with that of the delayed signal pulses. These coincidences provide evidence that the apparatus was producing entangled photons. After the apparatus was optimized, a double slit was placed in the path of the signal photons and used to search for heralded single photon interference. The results were inconclusive within measurement uncertainty.
A new fourth-order interference experiment has been carried out and analyzed theoretically in classical and in quantum terms. Two photons produced in the process of parametric down-conversion provide the two inputs to a Mach-Zehnder type of interferometer, while two photodetectors coupled to a coincidence counter measure the output. The coincidence rate, after subtraction of accidentals, exhibits a cosine variation with the optical path difference, in agreement with quantum mechanics, but in disagreement with a classical analysis. By contrast, when two coherent light beams from a He:Ne laser are used as inputs to the interferometer, no fourth-order interference is observed.
We demonstrate optical interferometry beyond the limits imposed by the photon wavelength using 'triggered' entangled photon pairs from a semiconductor quantum dot. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The results suggest that quantum interferometry may be possible using a semiconductor LED-like device. (C) 2007 Optical Society of America